The fired Two and a Half Men star -- whose character is reportedly killed off in the season premiere -- is currently paying Mueller $55,000 per month in child support. (The two are parents to 2-year-old twins Bob and Max.)

On Christmas day in 2009, Sheen allegedly held a knife to Mueller's throat and threatened to kill her. He later pled guilty on lesser charges and spent time in rehab, not jail. Since the incident, the two had lived apart, although their divorce wasn't officially granted until this past May. A source tells Us the parents have been "getting counseling and therapy [with] the boys" and Mueller, who has struggled with substance addiction, is "actually responding to treatment."

"She's totally gung-ho," the source adds. "He just wants his kids to be happy, and if that means they need to be a family, then he is up for it... The kids have suffered a lot, and he feels so guilty lately."

Sheen -- who also has two daughters with ex-wife Denise Richards and a 26-year-old daughter from a previous relationship -- "hates that he's made [his kids] so unhappy," according to a source. "No one believes anything he says, but the truth is he's been clean since February."